Inosine 5'-monophosphate dehydrogenase (IMPDH) is among the most attractive targets for drug design. IMPDH catalyzes the conversion of IMP to XMP with the concomitant reduction of NAD. This reaction is an unusual hydride transfer reaction; an adduct is formed between IMP and a Cys residue. Hydride is transferred to NAD, leaving a covalent E-XMP* intermediate. E-XMP* is hydrolyzed to produce XMP. The IMPDH reaction controls guanine nucleotide biosynthesis. IMPDH inhibitors have antiproliferative activity and the clinical utility of IMPDH inhibitors is well established in viral, immunosuppressive and cancer therapy. The significant differences between microbial and mammalian IMPDH's indicate that this enzyme is also a target for anti-infective chemotherapy. This proposal outlines a multidisciplinary investigation into the mechanism of IMPDH catalysis and inhibitor action. This fundamental information is important for the further development of IMPDH-based chemotherapy. The formation and hydrolysis of E-XMP* is of particular interest because the immunosuppressive drug pycophenolic acid binds to E-XMP*. Mycophenolic acid is also the only known species selective inhibitor of IMPDH. The origin of this species selectivity will be determined. The mechanism of IMPDH inhibition by oxanosine, an antibiotic agent, will be delineated. The structure of the enzyme-IMP complex of IMPDH from Borrelia burgdorferi, the causative agent of Lyme disease, will be refined. Preliminary results suggest that this structure will be more ordered than the currently available crystal structures of the apoenzyme and E*XMP complexes of IMPDH from Tritrichomonas foetus. The structures of IMPDH complexed with inhibitors will also be solved. These experiments will utilize steady state kinetic, pre-steady state kinetic, site-directed mutagenesis and x-ray crystallography.